1. Technical Field
The present invention relates to a method for manufacturing a surface acoustic wave element and the surface acoustic wave element manufactured by the method. In particular, it relates to a method for manufacturing the surface acoustic wave element provided with an interdigital transducer (IDT) inside of a recess formed in the center of the surface of a semiconductor substrate by a bank formed around the edge thereof, and a structure of the surface acoustic wave element.
2. Related Art
In recent years, mobile electronic apparatuses such as cellular phones become popular, and further, those apparatuses are required to be sophisticated and downsized. Therefore, electronic devices used for the mobile electronic apparatuses are also required to be downsized.
As for techniques to downsize such electronic devices, a functional device unit provided with a semiconductor element chip and a method for manufacturing the functional device are commonly known. The functional device unit includes an insulation substrate made of silicon (a Si substrate) having a recess on its surface and a ditch formed in an insulation film formed in the recess including the bottom and the side thereof, and on the top surface of the substrate, a wiring layer patterned in the ditch continuously from the bottom of the recess to the top surface via the side thereof, a semiconductor element chip which is flip-chip mounted between the wiring layers in the recess, and the recess being sealed with resin. (for example, JP-A-2002-33410 (No. 5, page 6, FIG. 2))
Alternatively, a method for manufacturing a surface acoustic wave element module is also known. In the method, the surface acoustic wave element (in a chip state) is arranged in a recess formed on a module substrate made of ceramic, and a sub substrate provided with a peripheral circuit is electrically coupled to the surface acoustic wave element. The surface acoustic wave element is sealed in the recess by the sub substrate. (for example, JP-A-5-152881(No. 3, page 4, FIGS. 2 and 3))
In JP-A-2002-33410 (No. 5, page 6, FIG. 2), the functional device unit is formed with a semiconductor element chip that is flip-chip mounted in a recess of an insulation substrate and sealed with resin. In this structure, even though the semiconductor element chip is accommodated in the recess of the substrate, the functional device unit needs to have a certain thickness for the bottom of the substrate corresponding to the semiconductor element chip and the sealing resin layer. Moreover, the size cannot be reduced to less than that of the substrate itself. Therefore, even if this semiconductor element chip is replaced with the surface acoustic wave element of the invention to be described below, it is still hard to attain a thin and small-sized surface acoustic wave element because of the reasons described above.
Further, when this semiconductor element chip is mounted on a functional device unit for electronic apparatuses or the like, it needs to be coupled with an outside circuit. Therefore, another step for coupling is necessary after the semiconductor element chip is flip-chip mounted on a substrate, thereby at least two mounting steps are required in total. The manufacturing process thus becomes longer. In addition, since a substrate to mount a semiconductor element chip is needed, it is hard to reduce the cost.
Further, according to JP-A-5-152881 (No. 3, page 4, FIGS. 2 and 3), an surface acoustic wave element (in a chip state) is arranged in a recess formed on a module substrate, and followed by sealing the recess having the surface acoustic wave element inside by the sub substrate, thereby protecting the surface of the surface acoustic wave element from contamination or the like. However, a module substrate, a surface acoustic wave element, and a sub substrate are individually formed and mounted on a surface acoustic wave element module, limiting the surface acoustic wave element module to be thin. In addition, increase of manufacturing steps such as mounting steps cannot be avoided.
Furthermore, for an IDT electrode forming region of the surface acoustic wave element, flatness and smoothness are highly required to obtain a predetermined resonance characteristic as it is known. However, it is assumed that the surface of a wiring pattern formed on the bottom of the recess on the module substrate according to JP-A-5-152881 (No. 3, page 4, FIGS. 2 and 3) described above is hard to be flattened and smoothed. Accordingly, accurate characteristics such as resonance frequency seem hard to obtain.